Seabed Mapping on the Brazilian Equatorial Margin: A Great Brazilian Barrier Reef?

The lack of integrated studies of the seafloor on the Brazilian margin has limited the detailed knowledge of this vast area. Although studies are being developed, they are generally restricted to isolated areas and there is no integration of different methods or different research groups. Previous studies have shown along the Rio Grande do Norte State (between 6^o and 4 ^o lat S) the presence of a chain of submerged sandstone outcrops at the 25 m isobaths, which marks the limit between mid- and outer shelf. It consists of laminated sandstone cemented by carbonate, often overgrown by a thin corallinaceae algae encrustations, and a large number of rhodolites and sponges, and is related to an ancient coastline. New researches have shown on the outer shelf the presence of living and fossil corals on the outer shelf following this chain. New satellite image processing and hydroacustical data (side scan sonar, multbeam bathymetry, sub-bottom profile), supported by diving, sediment samples collection, video transects and biological data (benthic cover, reef fish assemblages) provide unparalleled perspective on spatial distribution of environments on the northeastern Brazilian outer shelf. Combined, these different datasets allow effectively a description of the spatial variations of seafloor properties along the equatorial Brazilian shelf and show the presence of coral reefs at depths between 25 and 40 m depht, not yet described in the literature, thus expanding the knowledge regarding the coral distribution in Brazil. Identification of the species through the video-transects method showed that the species Siderastrea spp is predominant, followed by Montastraea cavernosa. These seabed features were recognized in different parts of the Brazilian equatorial outer shelf. This suggests that in the past, a barrier reef extends further south (at least to the state of Bahia, 13 degrees south latitude), and to the north (at least to the state of Amapa, 4 degrees north latitude). Moreover, their location in some places directly under the influence of an established and expanding oil industry opens new perspectives for environmental and similar studies.